Our systems have a predefined list of index pages they will attempt to load when someone visits your domain name, this can be useful as you can place two index files in your home directory and only one will be shown (the one with a higher priority).
For example: We often work with an index.php, particularly if we're using WordPress or some other PHP site. By uploading an index.html to the public_html directory as well the index.html will display instead (as it is a higher priority than index.php) and allow you to place an "under construction" page on your site without affecting your index.php (you can still visit index.php by defining it explicitly in your web browser).
On other occasions you may want to define the order of yourself, this is done by editing the .htaccess file in your public_html directory and adding the DirectoryIndex directive:
DirectoryIndex filename.html index.cgi index.pl default.htm
